Hold-down system



Dc., 23, 1941, R, TYLER HOLD-DOWN SYSTEM Filed Nov. 1e', l193@ 3 Sheets-sheet 1 BY f 3 Sheets-Sheet 2 Other Units all ' R. TYLER HOLD-DOWN SYSTEM Filed Nov. 16. 1938 laa Dec. 23, 1941.

JNVENTOR.

HANSEN TYLER BY f WATTORNEY.

Patented Dee. z3, 1941 UNITED '-sTA'r HOLD-DOWNl SYSTEM Ransom Tyler, Greenfield, Wis., assigner to-The Oilgear Company, Milwaukee, Wis., a corporation of Wisconsin Application November 16, 1938, Serial No. 240,668

This application is a continuation in part.V of application SerialY No. 94,034, led August 3, 1936, and now abandoned.

The invention relates to hydraulic mechanisms for exerting holding pressures upon external objects or for pressing one or more objects against one or more other objects, and it is particularly adapted for controlling the pressure between the rolls of printing presses of the type employed to' print paper or other material in colors.

A press of this type ordinarily has a lower roll -which rotates upon a stationary axis and an upper roll which is movable vertically and exertsa predetermined pressure upon the paper or other material as it passes between the rolls. In order that the printing may be clear and sharp, it is ordinarily necessary that the pressure betweenthe rolls be closely yregulated and in some instances be less than the pressure created by the weight of the upper roll assembly.

The presentinvention has an object to provide a hold-downV system in which a -plurality olhydraulically actuated hold-down unitsV may be energized by liquid from a common source and be capable of exerting independent holding forces without wasting'the motive liquid.

Another object is to provide such a system with means whereby the fluid pressure in one unit may be varied without varying the pressure in another unit.

Another object is to provide a hold-down system which is susceptible of close adjustment and control.

Another object is to provide a hold-down system lwhich maybe economically manufactured and operated.

Other objects and advantages will appear from the description hereinafter given of hold-down systems in which the invention is embodied.

The invention is exempliiied by the holddown systems illustrated schematically in the accompanying drawings in which the views are as follows:

Fig. l Iis a view showing o'ne form of the invention applied `to a printing press of the type employed to print in colors.

Fig. 2 is a detail view. f

Fig. 3 is a view showing another embodiment of the invention applied to a printing press.

Figs. 4 and 5 are views showing a control valve in `positions different'from that shown in Fig. 3.

Fig. 6 is a position different from Fig. 7 is a view showing view showing another valve in a that shown in Fig. 3.

a hold-down system similar to that shown in Fig. 3 but employing a single pumpinstead o1' two pumps.

Figs. 8 andy 9 are views showing a control valve in positions different from that shown in Fig; 6. For the purpose of "illustration, theinvention has been shown applied to a printing press but it is not limited to such use. Since the printing' I press per se forms no part of the present invention, it has not been shown in detail.

It is deemed suflicient to state herein that the press hasl one or more stands of rolls or press sections each of which includes a lower or impression roll I which has its trunnions journaled in stationary bearings 2 and 3, an upper roll 4 for pressing the material tol be printed against roll l, and two backing rolls 5 and 6 with a hydraulic hold-down oi a mechanism for raising Aupper roll assembly to which engage upper roll 4 4throughout the length thereof. Rolls 5 and 6 `havethe trunnions thereof journaled in a pair of vertically movable bearings 1 and ,8 which are shown as having slots formed therein to receive th trunnions of roll Vl so that roll `4 may move vertically 2a limited distance but, when bearings 1 and 8 re moved upward, they will carry the upper anfi backing,

rolls with them. l f

j Fia-11 In orderto show that the same or independen holding pressures may be applied to opposite ends of the same roll, two stands of rolls or press sections A and B have. been shown.

In each of sections A'and B, each of bearings 1 and 8 has connected thereto a piston or ram 9 which is fitted in a cylinder lll and forms theremotor. Cylinder I0 Il which Iorrnfs part and lowering the permit roll I to be changed or to be replaced by a larger or smaller roll. The mechanism for raising and lowering the upper roll assembly has not been illustrated as it forms no part of the invention. It is deemed sufficient to state that each rod ll is firmly held in a stationary position when the hold down is vshown carried by a rod motors are urging bearings 1 and 8 downward to press roll 4 against roll l.

Liquid for operating motors 9-ID may be supplied by a pump I5 whichhas the characteristic of delivering liquid at its full or adjusted volumetric rate until Vpump pressure reaches a predetermined maximum and then automatically reducing its stroke until it is delivering just suiiicient liquid to maintain that maximum pressure substantially constant. A pump of this type is shown in Patent No. 2,080,810. Pump I5 delivers its output into a pressure channel` I9 and has liquid returned to it through a return channel I1.

In order that the same or different pressure may be maintained in the several cylinders I9, each cylinder III or group of cylinders I9 is provided with a pressure control unit consisting of resistance valve or a relief valve I8 and a resistance valve I9 which open in opposite directions.

The valve I8 oi each unit has its inlet connected to a cylinder I9 or to a group oi cylinders I0 and its outlet connected to return channel I1, and it is adjusted to open at a pressure at least as great as the pressure desired to be maintained in that cylinder or group of cylinders. The resistance valve I9 of each unit has its inlet connected to pressure channel I9 and its outlet connected to a cylinder I8 or to a group of cylinders I and it is adjusted to open at a pressure equal to the diii'erence between the pressure in channel I and the pressure desired to be maintained in that cylinder or group of cylinders.

The hold down system may be arranged to cause the same or different pressures to be exerted upon opposite ends oi the upper roll or the assembly. vFor the purpose of illustration, each cylinder Il) in section A is shown as being connected by channels 2li to the outlet of a resistance valve I9 and through channel 20 and a channel 2I Ato the inlet of relief valve I8, while both of the cylinders in section B are shown as being connected by a branched channel 22 to the outlet of a single resistance valve I9 and through channel 22 and a channel 28 to the inlet of a single relief valve I8. 'Ihe arrangement is such that diierent pressing forces may, ii' desired, be applied to opposite ends of the upper roll assembly in section A while the same pressing force is applied to both ends of the upper roll assembly in section B.

In practice, the valves I9 and I9 of each unit are arranged in alinement in a single casing or in connected casings and are adjusted by a singie element but, for the purpose of illustration, they have been shown as being separate and Yprovided with a common adjusting member 24 rotation of which in one direction will increase the tension of the spring in valve I8 and decrease the tension of the spring in valve I9, thereby increasing the pressure required to open valve I8 and decreasing the pressure required to open valve I9, while rotation of member 24 in the opposite direction will decrease the tension of the spring in valve I8 and increase the tension oi' the spring in valve I9, thereby decreasing the pressure required to open valve I8 and increasing the pressure required to open valve I9. Therefore, the sum of the resistances of the two valves remains constant.

In order that the springs of valves I8 and I9 may be individually adjusted, member 24 is shown in Fig..2 as being provided with two separate adJusting screws 25 and 26 which are threaded, respectively, through the casings of valves I8 and I9 and into contact with the springs therein.

Screw 25 has a nut 21 fixed thereonnear its Aouter or upper end and that part of the screw beyond nut 21 is unthreaded and fitted in the bore of a hollow nut 29 which is fixed upon the outer orlower end of screw 26. Nuts 21 and 28 may be iixed for simultaneous rotation by means of a set screw 29 threaded through the wall of nut 28 into contact with the Unthreaded portion of screw 25.

When set screw 29 is loosened, adjusting screws and 29 may be turned independently of each other to individually adjust the resistances of valves I8 and I9 respectively. Then by tightening set screw 29, nuts 21 and 28 will be fastened together so that rotation of one will cause rotation of the other and thereby-cause the resistances of valves I8 and I9 to be simultaneously varied inversely to each other.

If the system is supplied with liquid from a constant pressure source, such as pump I5, and if it is desired to prevent the escape of liquid from the system during normal operation of the apparatus to which the system .is applied, the combined resistances of the valves I8 and I9 of each unit should be such that the pressure required to open both valves of each unit would be at least as great as the pressure at the source.

For example, if pump I5 were maintaining a pressure of 1000# per sq. in. in channel I8 and ii it were desired to maintain a pressure 800# per sq, in. in a certain cylinder I0 without loss of liquid from the system, the pressure control unit connected to that cylinder .should be so adjusted that the valve I9 thereof would open at a pressure of 200# per sq. in. and valve I8 thereof would open at a pressure of at least 800# per sq. in.

It is well known that a resistance valve will remain open at a pressure lower than the pressure at which it opens due to the fact that the area of the valve member exposed to the pressure at the inlet of the valve is greater after the valve opens than before it opens.

The additional valve member area exposed to the inlet pressure after the valve opens depends upon the width of the valve seat for the reason that, as soon as the liquid flowing through the valve passes the valve seat, its pressure drops to the pressure prevailing at the ,outlet side oi' the valve member. Therefore, if the valve is provided with a wide seat as is necessary to prevent it from becoming battered if the valve member has considerable movement due to large volumes being passed through the valve, the area exposed to the inlet pressure is considerably greater after the valve opens than before it opens and there will be a considerable difference between -the pressure at which the valve opens and the pressure at which it will close.

Since only very small volumes-are ever passed through valves I8 and I9, these valves merely "crack when liquid is passed therethrough so that there is substantially no impact against the seat when the valve closes. Therefore, each of valves I8 and 9 is provided with 4a very narrow hardened seat which is little more than a-sharp edge. Consequently, the differencel between the pressure at which the valve opens and the pressure required to hold it open `is so small that it is of no significance commercially and has been disregarded in the explanation given herein.

It would seem that the combined resistance of the two valves of each unit could be such that the pressure required toopen both valves would be Just equal to the pressure at the source but in practice it is foundlthat the combined resistance should be slightly higher so that each valve I8 will open at a -pressure which, i'or instance, is 10 to 50# higher than the pressure in the cylinder to which that valve I8 is connected.

As long as the Vpressure at theV source and the upward force acting upon the ram 9 of any holddown motor remains constant, there will be maintained in the cylinder I of that hold-down motor a substantially constant pressure which in the above example, would be approximately 800# per sq. in. The `pressure at the 'source might vary due, for instance, toinaccurate operation of pump |5, or the upward `force acting on ram 9 might vary due, for instance, to the paper breaking and then winding up on roll and thereby raising roll 4. If the pressure at the sourceshould increase or if the upward force lacting on ram 9 should vary, the pressure in cylinder I5 would be maintained withinnarrow limits, which in the above example, would be between approximately 800# and 810# to 850#.

The pressure in any hold-down motor and the holding force exerted thereby may thus be maintained substantially constant or kept within narf rowA limits, and the pressure in any hold-down motor and the holding force exerted thereby may thus be varied without affecting the pressure in any other hold-down motor or the force exerted thereby.

Figs. 3 t0 5.

`The hold-down system shown in-Fig. 3 differs primarily from the previously described system l in that it is providedwith hold-down motors which are capable'of moving the. upper roll assembly toward and from lower roll and with means fpr reducing the pressure between rolls I and 4to a minimum so that the pressure being exerted upon the material being printed willbe less than the pressure resultingV from the weight of the-upper roll assembly.

As shown, the vupper roll assembly has its bearing 1 connected bya rod 3| `to a piston 32 whichV is `itted in a stationary cylinder 33, and its bearing 5 connected by a rod 34 to a piston 35 which is iitted in a stationary cylinder 35.

Liquid for moving pistons 32 and 35 downward is supplied by a constant pressure pump 31 which discharges liquid into a supply channel 35 and has liquid returned to it through a return channel 39. Liquid for moving pistons 32 and 53 so that no liquid can flow to or from channel 51. When valve 45 is in the position shown in Fig. 4, communication between port 52 and channel 53 is blocked but port 52 is open to port 5| so that liquid can flow into channel 51 from supply channel 33. When valve45 is in the position shown in Fig. 5, communication between port 52 and port 5| is blocked but port 52 is open to channel 53 so that liquid can escape from channel 51 into return channel 39.

The pressure of the liquid supplied to the upper ends of cylinders 33 and 35is controlledby two pressure control units which are identical to the pressure control units shown in Figs. 1 and 2 and previously described. Consequently, these units have been indicated by the same reference numerals with the exponents a and b added thereto, and no detailed description thereof will be given.

As shown, channel 51 has its ends connected to two resistance valves i9* and i9b the outlets of which are connected, respectively, by channels 35 upward is supplied by a constant pressure pump which discharges into a supply channel 4|. `The liquid supplied by pumps 31 and 40 may be employed for operating other hold-down units by extending channels 35, 39 and 4| thereto.

The operation of hold-down motors 32-33 and 35-35 is undenthe control of a control valve which is fitted in a valve casing 45 and yleldingly retained in the position shown in Fig. 3 by a caged spring 41 arranged in a spring chamber 4B formed upon or attached to one end of casing 45. Valve 45 is adapted to be shifted from its central or neutral position as shown in Fig. 3 to the position shown inV Fig. 4 or to the position shown in Fig. 5 by one or the other of two solenoids 49 and 50 the cores of which are connected to valve 45 by suitabl'e valve stems.

Valve casing 45 has two' annular grooves or ports 5| and 52Y formed in the wall of its bore intermediate the ends thereof and the ends of its bore connected to a branched return channel 53 which is connected by a channel 54 to return channel 39. Port 5| is connected by channel 55 to supply channel 35, and port 52 is connected by a channel 55 to a channel 51 4intermediate the ends thereof.

When valve 45 is in the position shown in Fig. 3, it blocks communication between port 52 and port 5| and between port 52 and return channel cylinders 33 and 35 and liquid from pump 4|| 53* and 53 to the upper ends of cylinders 33 and 35. Channels 5|l'L and 55b are connected, respectively, by channels 59* and 59b to the inlets of two relief valves I5* and ib the outlets o! which have return channel 53 connected thereto. Valves I5'L and i9* may be simultaneously adjusted by means of an adjusting member 24e and valves l5b andi!b may be simultaneously adjusted by means of an adjusting member 24b as previously explained.

In order that pistons 32 and 35 may expel liquid freely `from upper ends of cylinders 33 and 35 when motive liquid is supplied to the lower ends of cylinders 33 and 35 and valve 45 is in the position shown in Fig. 5, two check valves 55* and 50b have the inlets thereof connected. respectively, to channels 55* and 55b and the outlets thereof connected, respectively, by channels 5i* and 5H to channel 51 intermediate the ends thereof.

If the press is to be employed at all times to print materials which require only a low pres- 'l sure between rolls .l and 4,v supply pipe 4| from pump 45 may be connected directly to the lower ends of cylinders 33 and 35 but, if a wide range of printing pressures is desired, means should be provided for connecting the lower ends of cylinders 33 and 35 either to pumpv 4l) or to exhaust.

As shown, the delivery of liquid to the lower ends of cylinders 33 and 35 is under the control of a valve 52 which is closely fitted in the bore of a valve casing53 having three' annular grooves or ports 54, 55 and 55 formed in the wall thereof. Port 54 is connected to supply channel 4| by channel 51, port 55 is connected to the lower ends of cylinders 33 and 35 by a branched channel 55, and port 55 and both ends of the bore of casing 53 are connected to a branched exhaust channel 59 which discharges into a suitable reservoir such as the sump of pump 40.

Assuming that the parts are inthe positions shown in Fig. 3 and that pumps 31 and 40 are running, the hold-down system will function as follows:

If it is desired to raise upper roll assembly, valve 45 is shifted to the position shown in Fig. 5 so that liquid can escape vfrom the upper ends of can flow to the lower ends of cylinders 33 and 35 and move pistons 32 and 35 upward. `'I'he liquid expelled from the upper end of cylinder 33 by piston 32 will flow through channel 53,

check vvalve 68, channels 6I, 51 and 56, valve casing 46 and channels 53 and 54 intothe return channel 33, and the liquid expelled from the upper end of cylinder 36 by piston 35 will flow through channel 58h, check valve 60", channels 6th, 51 and 56, valve casing 46 and channels 53 and 54 into return channel 39.

When it is desired to move the upper roll assembly downward, valve 45 is shifted to the position shown in Fig. 4 and'liquid from pump 31 will fiow through channels 38 and 55, valve casing 46 and channel 56 into channel 51. A part of the liquid entering channel 51 will flow through resistance valve I9 and channel 58a to the upper end of cylinder 33 and move piston 32 downward, and another part of the liquid will flow through resistance valve I 9b and channel 58b to the upper end of cylinder 36 and move piston 35 downward.

The liquid expelled from cylinders 33 and 36 during downward movement of pistons 32 and 35 may flow to pump 40 and be exhausted through the relief valve with which all such pumps are provided, it being understood that pump 40 is so.adjusted that pressure required to open its relief valve is but a few pounds higher than the predetermined pressure at which pump 40 reduces its stroke to maintain that predetermined pressure constant.

However, valve 62 may be shifted toward the right to or nearly to the position shown in Fig.

6 and then pistons 32 and 35 may expel liquid from the lower ends of cylinders 33 and 36 through channel 68 and valve casing 63 into exhaust channel 69 until just before the roll 4 engages lower roll I and then valve 62 should be shifted to the position shown in Fig. 3 to prevent roll 4 from striking a heavy blow upon` lower roll I.

After roll 4 is in contact with roll I, the pressure created by pump will act upon the lower faces of pistons 32 and 35 and create upward forces which will oppose the downward forces created by the weight of the upper roll assembly and by the pressure acting upon the upper faces of pistons 32 and 35. If the pressure control units are adjusted to cause sufficient drop in pressure between pump 31 and the upper'ends of the hold-down cylinders, the upward forces on pistons 32 and 35 will substantially counter-balance the downward forces thereon and the pressure between rolls I and 4 will be approximately zero. If the pressure control units are adjusted to cause a smaller drop in pressure between pump 31'and the upper ends of cylinders 33 and 36, the pressure between rolls I and`4 will be correspondingly greater.

If a high pressure between roll 4 and roll I is desired, valve 62 may be shifted to the position shown in Fig. 6 after roll 4 ls in contact with roll I. Pressure control units I8II8L and HIL-I9b will then function to maintain the holding pressure in the upper ends of cylinders 33 Figs. 7 to 9 The hold-down system shown in Fig. 7 differs from the system shown in Fig. 3 in that only one pump is employed, liquid is delivered from that pump to the lower ends of the hold-down motors through a pressure control unit similar to the pressure control unit previously described, and the fiow of liquid to and from the hold-down motors is controlled by a valve which differs somewhat from the control valve shown in Fig. 3. Since the system is otherwise the same as the system shown in Fig. 3, like parts have been indicated by like reference numerals and no further description thereof will be given.

As shown, liquid for operating hold-down motors 32-33 and 35-36 is supplied by pump 31 which, as previously explained, delivers its output into a supply channel 38 and has liquid returned to it through a return channel 39.

The flow of liquid to and from the hold-down motors is controlled by a valve 15 which is tted in a lvalve casing 16 and yieldingly retained in the position shown in Fig. 7 by a caged spring 11 arranged in a spring chamber 18 formed upon or attached to one end of valve casing 16. Valve 15 is adapted to be shifted from its central or neutral position as shown in Fig. '1 to the position shown in Fig. 8 or to the position shown in Fig. 9 by one or the other of two solenoids 18 and the cores of which are connected to valve 15 by suitable valve stems.

Valve casing 16 has four annular grooves or ports 8|, 82, 83`and 84 formed in the wall of its bore intermediate the ends thereof, and the ends of the bore are connected to a branched return channel which is connected by a channel 86 to return channel 39.

Port; 8I hasl channel 56 connected thereto, port 82 is connected to vsupply channel 38 by channel 81, and ports 83 and 84 are connected by branched channel 88 to the inlet of a resistance valve I9c the outlet of which is connected by branched channel 89 to the lower ends of both of cylinders 33 and 36.

Channel 89 is connected by channel 90 to the inlet of a check valve 9| the outlet of which is connected by a channel 92 to channel 88 intermediate the ends thereof. connected by a channel 93 to the inlet of a relief valve I8 the outlet of which is connected by a channel 94 to a return channel 39. Relief valve I8c and resistance valve I9c may be simultaneously adjusted by meansof an adjusting member 24 and form therewith a pressure control unit which is identical to the pressure control unit shown in Fig. 1 and previously described.

When valve 15 is in the position shown in Fig. '7, it blocks communication between ports 8l, 82, 83 and 84 so that no liquid can flow to or from the hold-down motors.

When valve 15 is shifted to position shown in Fig. 9, port 8l is open to exhaust channel 85 and port 82 is open to port 84 so that liquid from pump 31 will flow through channels 38 and 81, valve casing 16, channel 88, resistance valve` I9c and channel 89 to the lower ends of cylinders 33 and 3'6 and cause pistons 32 and 35 to move upward and raisethe upper roll assembly.

Piston 32 will expel liquid from the upper end of cylinder 3,3 through channel 58, check valve 6I!a and channel ISIa into channel 51, and piston 35 will expel liquid from the upper end of cylinder 36 through channel 58h, check valve 6I)b and channel 6Ib into channel 51. From channel 51,

the liquid expelled from cylinders 33 and 36 by pistons 32 and 35 will flow through channel 56,

Channel 89 is also' valve casing 16 andchannels 85 and 66 into return channel 39.

Whenvalve 1B is shifted t0 the position' shown in Fig. 8, communication between channel 65 and the valve ports is blocked so that no liquid can fromisaid source tov said hold-down means including a resistance valve having a resistance substantially equal to` the difference between the pressure at said source and the pressure desired escape into return channel 39, andvports 6I and 83 are open to port Ill-so that liquid from pump 31 will at first flow through channels 38' and 81, valve casing 16 and channelr 56 into channel 51. From channel 519. part of the liquid will flow through resistance valve I9* and channel 58 to the upper end of cylinder'33 ,and move piston 32 downward, and another part of the liquid will flow through resistance valveA I9'J vand channel 8b to the upper end of cylinder 36 and move piston 35 downward. thereby causing the upper roll assembly to move downward.

Pistons 32 and 35 in moving downward will expel liquid from the lower ends of cylinders 33 and 36 through channels 69 and 90, vcheck valve 9|, channels 92 and 88 and valve casing 1 6 into channel 56 where it will join the liquid supplied to channel 56 by pump 31 so that pistons 32 and 35 are moved downward at high speed due to the fact that pump 31 need supply only a volume of liquid equal tothe displacement of rods 3| and 34.

When upper roll 4 engages lower roll I; further downward movement of pistons 32 and 35 is arrested and pressure will extend through the above described channels to cylinders 33 and 36.

The pressure in the upper end of cylinder 33 is maintained within narrow limits by resistance valve I9EL and relief valve I8, and the pressure in the upper end of cylinder 36 is maintained within narrow limits by resistance valve I9 and relief valve IBb as previously explained.

Since liquid could not escape from the lower ends of cylinders 33 and 36 through check valve 9| against the higher pressure prevailing in channel B6, relief 'valve I8 is provided so that liquid can flow from the lower ends of cylinders 33 and 36 through channels 69 and 93, relief valve Ill` and channel 94 into return channel 39 whenever the pressure in the lower ends of the cylinders exceeds the resistance of relief valve I6. Consequently, the pressure in the lower ends of cylinders 33 and 36 is maintained within narrow limits by relief valve IBc and resistance valve |96.

The upward forces exerted upon pistons 32 and 35 by the liquid in the lower ends of the cylinders oppose the downward forces exerted upon the pistons by the Weight of the upper roll assembly and by the liquid in the upper ends of the cylinders so that the pressure between rolls I and 4 is equal to the difference between the upward and `downward forces.

Since the pressure in each end of each cylinder is maintained within very narrow limits as previously explained, the pressure between rolls I and 4 may be maintained within very narrow limit and, by proper adjustment of the several pressure control units, the pressure between rolls I and 4 may be varied between zero and a predetermlnedi maximum pressure.

The hold-down system herein set forth is susceptible of various other modifications and adaptations without departing from the scope of the invention as hereinafter claimed.

The invention is hereby claimed as follows:

l. A hold-down system, comprising a source of constant pressure motive liquid and a plurality of hold-down units connected to said source. each of said units comprising hydraulically Vactuated hold-down means, means for supplying liquid in said hold-down means, and means for permitting liquid to escape from said hold-down means including a relief valve adapted to open at a pressure at least as great as said desired pressure, the sum of the reslstances of said valves being such that the pressure required to open both valves simultaneously is greater than the pressure at said source.

2. A hold-down system, comprising a source of constant pressure motive-liquid and a plurality of hold-down units connected to said source, each of said units comprising hydraulically'actuated hold-down means, means for supplying liquid from said source to said hold-down means includinga resistance valve for causing a drop in pressure between said source and said hold-down means, means for permitting liquid to escape from said hold-down means including a relief valve connected thereto, and a single means for simultaneously varying the resistances of said valves inversely to each other, the sum of the resistances of said valves being such that the preshold-down means, means for supplying liquid from said source to said hold-down means including a resistance valve for causing a drop in pressure between said source and said hold-down v means, means for permitting liquid to escape from saidV hold-down means including a relief valve connected thereto, means for individually adjusting each of said valves to vary the resist'- ance thereof, and meansfor operating both of said adjusting means simultaneously tol vary the resistance of said valves inversely to each other,

the sum of the resistances of said valves being` at all times such that the -pressure required to open both valves simultaneously is greater than the pressure at said source.

4. The combination, with a source of motive liquid having a substantially constant pressure, of a cylinder, means connecting said cylinder to said source and including a resistance valve adapted to close at a pressure substantially equal to the difference between the'pressure at said source and the pressure desired in said cylinder, means for permitting liquid to escape from said cylinder including a relief valve adapted to open at a pressure at leastas great as said desired pressure, and means for simultaneously adjustlng both of said valves to vary the resistance of each while maintaining the combined resistance of the two valves substantially constant.

5. In a machine having a first roll rotatable upon a stationary axis and a second roll movable toward and fromsaid rst roll, the combination of hydraulic means for urging said second roll against said iirst roll, a constant pressure pump, means connecting 'said hydraulic means to said pump including a resistance valve having a resistance equal to the difference between pump pressure and the pressure desired in said hydraulic means, means connecting said hydraulic means to an exhaust including a relief valve adjusted to open at a pressure at least as great as the pressure desired in said hydraulic means, and means for simultaneously increasing the resistance of one of said valves and decreasing the resistance or the other of said valves to thereby vary the pressure in said hydraulic means while maintaining the combined resistance of said valves substantially constant.

6. The combination, with a plurality of movable members and a source of motive liquid having a constant pressure, of a hold-down unit for each of said members, each of said units comprising a piston and a cylinder, uid channels l connecting the upper end of said cylinder to said source and having connected therein a resistance valve having a resistance substantially equal to the difference between the pressure at said source and the pressure desired in said cylinder', uid l5 channels connecting the upper end of said cylinder to an exhaust and having connected therein a relief valve which permits liquid to escape from said cylinder upon the pressure therein exceeding said desired value, and means for supplying to the lower end of said cylinder'liquid at a predetermined pressure which exerts an upward\ force on said piston to counterbalance in part the weight of said member and the downward force exerted upon said piston by the liquid in the upper end of said cylinder.

7. The combination, with a plurality of movable members and a source of motive liquid having a constant pressure, oi' a hold-down unit for each of said members, each of said umts comprising a piston and a cylinder, iluid channels connecting the upper end of said cylinder to said source and having connected therein a resistance valve having a resistance substantially equal to the difference between the pressure at said source and the pressure desired in said cylinder, iluid channels connecting the upper end of said cylinder to an exhaust and having connected therein a relief valve which permits liquid to escape from said cylinder upon the pressure therein exceeding said desired value, means for supplying to the lower end of said cylinder liquid at a predetermined pressure which exerts an upward force on said piston to counterbalance in part the weight of said member and the downward force exerted upon said piston by the liquid in the upper end of said cylinder, and valve means for controlling the ilow of liquid to and from said cylinder and operable to cause said liquid to raise or lower said piston.

8. The combination, with a plurality of movable members and a source of motive liquid having a constant pressure, of a hold-down unit for each of said members, each of said units comprising a piston and a cylinder, fluid channels connecting the upper end of said cylinder to said source and having connected therein a resistance valve having a resistance substantially equal to the difference between the pressure at said source and the pressure desired in said cylinder, o fluid channels connecting the upper end of said cylinder to an exhaust and having connected therein a relief valve which permits liquid to escape from said cylinder upon the pressure therein exceeding said desired value, means for simultaneously adjusting both of said valves to vary the resistance of each while maintaining the total resistance of the two valves substantially constant, and means for supplying to the lower end of said cylinder liquid at a predetermined pressure which exerts an upward force on said piston to counterbalance in part the Weight of said member and the downward force exerted upon said piston by the liquid in the upper end o! said cylinder.

9. The combination, with a plurality of movable members and a source of motive liquid having a constant pressure, of a hold-down unit for each of said members, each of said units-comprising a piston and a cylinder, fluid channels connecting the upper end of said cylinder to said source and having connected therein a resistance valve having a resistance substantially equal to the difference between the pressure at said source 'and the pressure desired in said cylinder, fluid channels connecting the upper end of said cylinder to an exhaust and having connected therein av relief valve which permits liquid to escape from said cylinder upon the pressure therein exceeding said desired value, means for simultaneously adjusting both of said valves to vary the resistance of each while maintaining the total resistance of the two valves substantially constant. means for supplying to the lower end of said cylinder liquid at a predetermined pressure which exerts an upward force on said piston to counterbalance in part the weight of said member and the downward force exerted upon said piston by the liquid in thgnipper end of said cylinder, and valve means for controlling the ow of liquid to and from said cylinder and operable to cause said liquid to raise or lower said piston.

l0. In a hold-down system having a plurality of units, each including a piston and a cylinder, the combination of a source of substantially constant pressure liquid, a channel for supplying liquid from said source to said units, a channel for.

returning liquid from said units to said source, a third channel, a control valve for connecting said third channel to said supply channel or to said return channel alternatively, each of said `cylinders having a fourth channel connected to one end thereof, a resistance valve having its inlet connected to said third channel and its outlet connected to said fourth channel, a relief valve having its inlet connected to said fourth channel and its outlet connected to said return channel, the pressure required to open said resistance and relief valves simultaneously being greater than the pressure at said source, an oppositely opening check valve connected in parallel with said resistance valve for permitting liquid to escape from said cylinder upon said control valve being adjusted to connect said third channel to said return channel, and means for supplying liquid to the other end of said cylinone end thereof, a resistance valve having its inlet connected to said third channel and its outlet connected to said fourth channel, a relief valve having its inlet connected tov said fourth channel and its outlet connected to said return channel, the pressure required to open said resistance and relief valves simultaneously being greater than the pressure at said source, an oppositely opening check valve connected in parallel with said resistance valve for permitting liquid to escape from said cylinder upon said control valve being adjusted to connect said third channel to said return channel, a second source of substantially said third channel to said supply channel or to said return channel alternatively, each of said cylinders having `a fourth channel connectedto one end thereof, a resistance `valve having its inlet connected to said third channel and its outlet connected to vsaid fourth channel, a relief valve having its inlet connected to said Vfourth channel and its outlet connected to said return channel, the pressure required to open said resistance and relief valves simultaneously being greater than the pressure at said source, .an oppositely opening check valve connectedin parallel with said resistance valve for permitting liquid to escape from said cylinder upon'said control valve being adjusted to connect said third channel to -saidxreturn channel, and means for supplying liquid from saidsource to the other' end of said cylinder including a resistance valve for reducing the pressure of said liquid to a predetermined value and a relief valve for permitting liquid to escapelfrom said other cylinder end upon the pressure therein exceeding a predeter- 13'. In ahold-down system having a plurality of units, each including a piston and a cylinder, the combination of a source of substantially constant pressure liquid, a channelfor `supplying liquid from said source to said units, a channel for returning liquid from said units to said source, a third channel, a control valve for connecting said third channel to said supply channel or to said return channel alternatively, each of said cylinders having a fourth channel connected to one end thereof, a resistance valve having its inlet connected to said third channel and its outlet connected to said fourth channeharelief valve having its inlet connected to said fourth channel and its outlet connected to said return channel, the pressure required to open said resistance and relief valves simultaneously being greater than the pressure at said source, means for simultaneously adjusting said resistance and relief valves to vary the resistance of each while maintaining the total resistance of the two valves substantially constant, an oppositely opening check valve connected in parallel with said rel, sistance valve for permitting liquid to vescape from said cylinder upon said control valve being adjusted to connect said third channel to said return channel, and means for supplying liquid to the other end of Vsaid cylinder at a substantially constant pressure.

14. In a hold-down systemhaving a plurality of units l. each including a piston and a cylinder, thecombinationrof a sourcerof substantially constant pressure liquid, a channel for supplying liqv uid "from said source to saidrunits, a channel for vreturning liquid from said units to said source, a third channel| a control valve for connecting said third channel to said supply channel or to vsaid returnchannel alternatively, each of said cylinders having a `fourth channel connected to -one end thereof, a resistancervalve having its linlet connected to said third channel and its outlet connected to said fourth channel, a relief valve havingits inlet connectedtorsaid fourth chan- ,neland its outlet connected .to .saidreturnv channel, the "pressurerequired to open said resistance and relief valves simultaneously being greater than the pressure -at said source, means for simultaneously adjusting said resistance and relief l `valves to vary the resistance `of each while maintainingtl'le total resistance of the two valves substantially constant, an oppositely opening check valve connected in parallel with said resistance valve for permitting liquid vto escape from said' cylinder Yupon said control valve being adjusted to connect said `third channel to said return channel, a second source of substantially Vconstant pressure liquid, and means including a valve forv connecting the other end of said cylinder either .to said second source or to an exhaust.

15, In a holddown system having a plurality of units, `each including a piston and a cylinder, thecombination of a source of substantially constant pressureliquid, a channel for supplying liquid from said source to said units, a channel for returning liquid from -sad units to said sourcea third channel, a control vaivefor `connecting said third channel to said supply channel or to said return channelalternatively, each of lsaid cylinders having Va fourth channel connected to one end thereof, a resistancel valve having its inlet connected to said third channel and its outlet connected to said fourth channel, va relief v alve vhaving its inlet connected to said `fourth channel and its outlet connected vto said return channel, the pressure required to open said :re-

sistance and relief valves simultaneously being greaterthan the pressure at rsaid source, means for simultaneouslyadjusting said resistance and relief valves to vary the resistance of each while maintaining the total resistance of the two valves substantially constant, `an oppositely opening check valve connected in parallel with ysaid resistance` valve for permitting liquid lto escape from said cylinder upon said control valve being adjusted to connect said third channel to said return channel, and means for supplying liquid from said source to the other end of said cylinder including a resistance valve for reducing the pressure ofsaid liquid to a predetermined value and a relief valve for permitting liquid to escape from .said other cylinderend upon the pressure therein exceeding a predetermined value.

RANSOM TYLER. 

